Often it is said that vast long-range economic and social forces, not the efforts of leading individuals alone, make history. The course of World War II denies this seemingly rational thesis. Hitler began World War II; he and his principal adversaries—Britain’s Winston Churchill, America’s Franklin Roosevelt, and Russia’s Joseph Stalin—determined the conflict’s course and outcome. While the latter two effectively won the war in 1945, Churchill played a significant role by not losing it in 1940 and 1941. If Churchill had not become prime minister of Britain in 1940 (or had the United States elected an isolationist, such as Herbert Hoover, as president), Hitler would have won. More than anyone else, Churchill understood that the entire British war effort depended on the United States. Roosevelt knew this, too, but it took him some time to recognize the full implications of the relations between the two nations. Among the war leaders, the Churchill-Roosevelt relationship would evolve into one of the most complex and decisive of the war.

Churchill and Roosevelt had met only once before, 22 years earlier in London, when the latter was assistant secretary of the navy. Roosevelt remembered the encounter, but Churchill had forgotten, which mildy irritated the president. Some advisors to the president, as well as his wife, Eleanor, were leery of Churchill, believing him to be a Tory imperialist who was too old and fond of drink. Much of this assessment would change, but not right away.

Early in 1939 Roosevelt had begun privately contacting political figures in Europe who appeared willing to stand up to Hitler. When installed as first lord of the admiralty in September, Churchill knew this and instigated his own confidential correspondence with the president; Churchill made it known to Prime Minister Neville Chamberlain and his foreign secretary, Lord Halifax. Then, on May 10, 1940—the day that Hitler invaded Belgium, the Netherlands, Luxembourg, and northern France—Churchill became prime minister of Great Britain. His relationship with Roosevelt took on new dimensions.

Over the course of the war, Churchill and Roosevelt would exchange nearly 2,000 letters and telegrams, an unprecedented level of correspondence between two national leaders. In 1940, when Hitler came close to winning the war, Churchill and Roosevelt mostly communicated in writing. (While phone conversations did take place, Churchill preferred not to communicate this way.)

In May 1940 only the prime minister and a few others fully grasped the bleakness of Britain’s prospects—and that events were moving fast. Churchill’s letters and speeches clearly reveal his determination to rouse his compatriots and fight. “I have nothing to offer but blood, toil, tears, and sweat,” he famously said in his first speech to Parliament as prime minister on May 13. These words had little effect at the time, making no particular impression on Franklin Roosevelt. Unlike Churchill, the American president did not know that France might soon collapse under German pressure.

That spring, as news from France grew more dreadful, the United States began to occupy Churchill’s attention. Two days after his speech to Parliament, while preparing for a flight to Paris, he drafted a long letter to Roosevelt. “If necessary,” he wrote, “we shall continue the war alone and we are not afraid of that,” hinting that France might soon fall. “But I trust you realize, Mr. President, that the voice and force of the United States may count for nothing if they are withheld too long. You may have a completely subjugated, Nazified Europe established with astonishing swiftness, and the weight may be more than we can bear [author’s italics]. All I ask you now is that you should proclaim nonbelligerency, which would mean that you would help us with everything short of actually engaging armed forces. Immediate needs are . . . .” There followed a list.

On the following day, Roosevelt responded that he could not commit to any course of action without congressional approval. At the time, there was widespread isolationist sentiment among the American people; he was about to run for an unprecedented third term and could not risk alienating a large constituency. Nor was Roosevelt yet inclined to trust the prime minister. Churchill thanked Roosevelt anyway in a message of five sentences, two of which are telling: “I do not need to tell you about the gravity of what has happened. We are determined to persevere to the very end, whatever the result of the great battle raging in France may be.”

Two days later Churchill wrote again to Roosevelt, including these ominous words: “Members of the present Administration would likely go down during this process should it result adversely, but in no conceivable circumstance will we consent to surrender. If members of the present administration were finished and others came in to parley amid the ruins, you must not be blind to the fact that the sole bargaining counter with Germany would be the Fleet; and if this country was left by the United States to its fate no one would have the right to blame those then responsible if they made the best terms they could for the surviving in-habitants. . . . Excuse me, Mr. President, for putting this nightmare bluntly. Evidently I could not answer for my successors who in utter despair or helplessness might well have to accommodate themselves to the German will. However, there is happily no need at present to dwell on such ideas.” Few understood at the time what Churchill knew implicitly: Roosevelt believed that should Britain fall, its fleet would cross the Atlantic for Canadian and American ports.

Churchill was not threatening Roosevelt. The prime minister was looking ahead toward a looming abyss. Should the Germans conquer England, or force a future British government to surrender, the removal of the fleet to the Western Hemisphere would mean little. Roosevelt did not realize that the mechanization of large armies, which could now move fast on land with the support of air power, had cut into the centuries-old, critical importance of sea power.

Between May 24 and 28, as Britain’s only army was trapped against the Dunkirk coast, Churchill struggled with Halifax, who wanted to ascertain—not unreasonably—what Hitler might be willing to offer Britain if they asked for terms. At the same time, Roosevelt and Mackenzie King, Canada’s prime minister, secretly discussed particulars about the eventual arrival of the Royal Navy in North America. Late on May 28, Churchill prevailed against Halifax in secret sessions of the War Cabinet. Halifax suggested another appeal to Roosevelt. Churchill argued that such a gesture would prove useless.

One disaster followed another. While the British army at Dunkirk was largely saved, the Germans marched into Paris a fortnight later, and France fell. Only now did Churchill invoke the United States. In a speech, he said: “If we fail, then the whole world, including the United States [author’s italics], and all that we have known and cared for, will sink into the abyss. . . .” U.S. support for intervention in Europe remain mixed. In 1940 Herbert Hoover, Joseph Kennedy, Charles Lindbergh, and other influential Americans opposed entering the war. Roosevelt would come to understand Churchill’s perspective, but not yet.

Churchill warned that Americans would stand alone against a Nazified “United States of Europe” if England fell

The desperate French Premier Paul Reynaud had begged FDR to enter the war, but the president declined. On June 15 Churchill wrote to Roosevelt again, urging him to consider the consequences: “Although the present Government and I personally would never fail to send the Fleet across the Atlantic if resistance was beaten down here, a point may be reached in the struggle where the present Ministers no longer have control of affairs and when very easy terms could be obtained for the British Islands by their becoming a vassal state of the Hitler Empire. . . . If we go down you may have a United States of Europe under Nazi command far more numerous, far stronger, far better armed than the New World.”

One of Churchill’s main concerns—second in importance only to preparations against a German invasion—was to make a single point clear to Roosevelt, something he could never mention in public, certainly not in a speech: the British Fleet would not go to the United States or Canada under any circumstances. On June 24 Churchill wrote to the Canadian prime minister: “There is no question to make a bargain with the United States . . . our despatch of the Fleet across the Atlantic should the Mother Country be defeated. . . . I shall myself never enter into any peace negotiations with Hitler, but obviously I cannot bind a future Government which, if we were deserted by the United States and beaten down here, might very easily be ready to accept German overlordship and protection. It would be a help if you would impress this danger upon the President.”

Two days later he cabled the same sentiments to Lord Lothian, the British ambassador to Washington, who had suggested that Churchill make another speech. “No doubt I shall make some broadcast presently,” Churchill answered, “but I don’t think words count for much now. Too much attention should not be paid to eddies of United States opinion. Only force of events can govern them.” He repeated that if the Germans could invade and subdue Britain, “the British Fleet would be the solid contribution with which [a] Peace Government would buy terms. We know [the] President is our best friend, but it is no use trying to dance attendance upon Republican and Democratic Conventions.”

Dependent on the United States and the Soviet Union, Churchill understood that victory against the Germans could cost Britain its empire. While this certainly concerned him, he knew also that the imperial ambitions of the British people had been fading even before the war. Churchill calculated that defeating Hitler and saving Britain from becoming, at best, a junior partner of Germany was worth the price of turning much of the empire over to America. As 1940 proceeded and the “finest hours” came and went, Churchill gradually became ever more confident that he and Britain would not lose the war. He realized that Hitler was far from losing it, too.

Then the prime minister surprised and impressed Roosevelt with his improbable decision to attack the French fleet. In 1940 France boasted the world’s third largest navy, far bigger than Germany’s. While Hitler put less store in naval power than did Roosevelt, he knew that he must neutralize this powerful fleet. Hitler consequently set special conditions for an armistice with the new French government so as to avoid the prospect that the French fleet, including some of the finest battleships in the world, would sail for England or the United States. The armistice directed that French warships would not be transferred to the German navy but would remain in French ports under German surveillance. Three of these battleships were docked in the French Algerian port of Oran, known today as Mers-el-Kébir. Churchill ordered the British commander-in-chief in the Mediterranean to present his French counterpart with an ultimatum: if the ships did not set sail for Britain or cross the Atlantic to the Western Hemisphere, British warships would sink them.

Churchill admitted that the prospect was “heartbreaking” but necessary. The honorable French admiral, Marcel-Bruno Gensoul, declined the offer. Late in the afternoon of July 3, the British fleet opened fire, sinking one battleship and damaging another, while the third escaped. Twelve hundred and fifty French sailors died during this strange battle, which lasted less than an hour. The action at Oran showed Roosevelt that the prime minister had no intention of giving up. Less than a month later, the president began preparations for his first definitely unneutral step—the promise of transferring 50-odd American destroyers to Britain.

The British fleet opened fire, sinking one French battleship and killing 1,250 sailors

Roosevelt moved closer to a virtual alliance with Britain. Intense German bombing did not cow the British, nor did the German air force win the Battle of Britain in the air. By the end of the year, Roosevelt declared that the United States was becoming the “arsenal of democracy,” offering and lending every sort of sea, air, and land equipment to Britain. A few months later, American forces moved into Greenland and other islands in the western Atlantic where an undeclared war between American and German naval forces developed.

Hitler invaded Russia in June 1941. The German bombing of London now diminished, due to the reassignment of the Luftwaffe to targets in Russia, but did not cease. The prospect of Allied victory receded into darkness. British factories cranked out machines, tanks, and airplanes; many of them were shipped through black nights and ice to Russia, but to little avail. The British civilian population was becoming more hard pressed by the day. Churchill faced an awful prospect: if the Germans conquered Russia, what could he and Roosevelt do? The two had met off Newfoundland in August, but the president still could not commit the United States to war. The day before he invaded the Soviet Union, Hitler ordered all German ships and submarines patrolling the Atlantic not to fire at American vessels under any circumstances, even in self-defense. Hitler knew that an incident could provide the pretext that Roosevelt needed.

Stalin, much as Churchill had, recognized the implications of Hitler’s German powerhouse. What he said to Roosevelt’s emissary Harry Hopkins at the end of July 1941 still holds the power to stun nearly 70 years later: Germany’s might was such that Russia and Britain together might not be able to break it. What would change everything, he told Hopkins, would be an American declaration of war against Germany. Churchill thought the same. In the autumn of 1941, the prime minister often said that if forced to choose between a total American stoppage of arms and aid to Britain and a sudden American declaration of war on Germany, he would choose the latter without hesitation.

The attack on Pearl Harbor on December 7, 1941, brought America into the war, but it did not change everything—at least not right away. In the evening of that infamous day, Averell Harriman had dined with the Churchills at Chequers, the country estate of prime ministers; later he recalled that his host had been “tired and depressed.” Churchill spoke hardly at all and sat glumly “with his head in his hands” for long minutes. In The Second World War , the reader palpably feels Churchill’s relief the moment his butler brought news of Pearl Harbor. His first thoughts were that Britain would live and the empire survive. He immediately phoned Roosevelt. On that dark winter night, the news fanned the flames of his spirit. While he—and Roosevelt—had known that the Japanese intended an attack, they did not know where. Churchill had remained fearful that a Japanese attack against only British possessions and bases in the Far East might not provide sufficient ammunition for Roosevelt to ask Congress to declare war on both Japan and Germany.

Churchill now knew that the war could not be lost; but did he think that the war—first and foremost, the war against Hitler—was now won? The next morning he traveled to Parliament and told a packed house that, “It is of the highest importance that there should be no underrating of the gravity of the new dangers we have to meet, either here or in the United States.” Two days later he said that the British must “KBO,” or “keep buggering on.” These three words, while less grave than “blood, toil, tears, and sweat,” delivered essentially the same message.

Britain’s dependence on the United States kept Churchill from criticizing his ally publicly and even in private for the most part. Well after the war he omitted, deleted, and left unmentioned almost all of his concerns and anxieties about Roosevelt and American military leaders during the last decisive year of the war, 1944–45. He did this, he later explained in a letter to Eisenhower, who was about to assume the presidency in 1952, with the American-British friendship in mind. (Eisenhower failed to appreciate this favor and treated Churchill with inexcusable disdain then and thereafter.)

Many believe that Churchill’s disagreements with Roosevelt during the last year of the war lay with the American propagation of the piecemeal dismantling of the British Empire, especially in Asia. That is not the case. The main cause was the Soviet Union—its ambitions and heavy presence in so much of Europe after the war. Churchill recognized Stalin’s dangerous aims well before the Americans. He saw the looming dangers and potential tragedies of the postwar world.

Six years after the war ended, he published “Triumph and Tragedy,” the sixth volume of his The Second World War . The triumph over Hitler may not have had to lead to a tragedy, but it did lead to a division of Europe, an iron curtain, and the development of a terrible cold war between the Soviet Union and the West. No American or Russian president or general would call the outcome of the war in Europe a tragedy. Churchill did.